The present invention relates to a differential amplifier. More specifically, it relates to an RF differential amplifier, a differential converter provided with the differential amplifier, and a mixer provided with the differential converter.
The fields of radio communications (e.g., mobile communications) and wired communications (e.g., cable TV) have imposed a demand for excellent signal linearity (low distortion) in an RF transmitter/receiver circuit, in order to improve data transfer efficiency. To prevent interference between RF signals and highly integrated circuits, it is preferable to form a differential circuit on a printed circuit board (PCB), especially on a Radio Frequency Integrated Circuit (RFIC). The frequency mixer typically used in an integrated circuit is of the Gilbert-Cell type, which includes a differential circuit. Accordingly, an excellent linearity (low distortion) has been required also to the differential circuit.
FIG. 1 is a schematic circuit diagram of a prior art differential amplifier 10. The differential amplifier 10 includes an emitter-coupled differential pair 11. The differential pair 11 includes a pair of NPN transistors Q1, Q2 with the emitters connected to each other. The emitters of the NPN transistors Q1, Q2 are connected to a current source 12 that supplies a bias current. The bases of the transistors Q1, Q2 are connected to input terminals 13, 14, respectively. The collectors of the transistors Q1, Q2 are connected to output terminals 15, 16, respectively.
Since the differential amplifier 10 operates by switching the bias current, the signal linearity is limited by an inner potential of the PN junction, and its maximum output current is limited by the bias current. Therefore, the prior art circuit usually employs a method of improving the linearity at an input conversion by means of the emitter degeneration at the expenses of the gain and the noise characteristics, which are as important as the distortion characteristic. This method does not improve the linearity of the output power (output current), however.
Further, there arises a problem in the balance of the differential outputs. The problem is related to the balance of amplitudes of the differential outputs and the balance of phase errors thereof. The balance of the differential outputs is in connection with the balance of the differential input signals. The balance of the differential input signals has considerable influence on many characteristics of the differential circuit, such as the isolation characteristic and the spurious characteristic of the double balanced type Gilbert-Cell mixer, and the orthogonal accuracy in the orthogonal phase shifter comprising a differential CR network. In the case of the orthogonal phase shifter comprising the differential CR network, a spurious component produced by the distortion affects the orthogonal accuracy. And these important characteristics all have an effect on the communication system.
Moreover, in the prior art differential pair, a slight shift of the balance of the RF input signals results in an unbalance to the output signals. The differential pair is used in an input circuit of the Gilbert-Cell mixer, and is used in a differential signal buffer amplifier as well. Accordingly, it is important to improve the balance of the output signals.
Additionally, the ICs are supplied with the differential signals through a balun. The balun includes an RF laminated hybrid balun that is used in the RF circuit of a transceiver. However, the RF laminated hybrid balun has a narrow band characteristic, which makes it difficult to attain a sufficient balance.